The generation of electric power from fossil fuel is receiving careful scrutiny with respect to its impact on the environment. The impact can be measured in terms of heat and pollutant emissions to the biosphere. Emission of waste heat is an unavoidable result of thermodynamics, but the specific quantity of heat emitted per kilowatt of electricity generated can be minimized by improvement of the thermal efficiency of the power generating cycle. Pollutant emissions can be minimized preferably by treatment of the fuels prior to combustion, although post combustion treatment is also technically and, sometimes, economically feasible.
While coal, and particularly high-sulfur, high-ash coal, is considered to be a source of potentially high pollution, improved technology to convert coal to desulfurized, ash-free gas is one feasible means of avoiding deleterious emissions. The technology is continuing to be developed. The processes used for this process are designed to extract the sulfur from the raw gas and to recover the sulfur compounds as elemental sulfur. The ash is recovered as cinders or granulated slag, leaving only a clean gas that is benign in environmental impact for use as fuel.
Gasification of coal normally produces a raw gas which comprises largely carbon monoxide and hydrogen, admixed with lesser quantities of carbon dioxide and methane. In addition, gaseous sulfur compounds, notably H.sub.2 S and COS, together with ammonia, elemental nitrogen, hydrogen cyanide and argon will be found at relatively low concentrations. Removal of the sulfur compounds, ammonia and cyanide can be accomplished economically, thus preventing the emission of sulfur compounds and reducing nitrogen oxides in the combusted gas.
Thermal efficiency of the power generation cycle can be significantly increased by combining the gas combustion turbine as a "topping" cycle with a "bottoming" steam turbine. The steam turbine may be either totally condensing, partially condensing, or non-condensing depending on the capability of the overall system to utilize low-pressure steam.
Whereas an installation where electric power is the sole product will have relatively minor uses for extraction steam, a multi-product plant can find many uses for steam at pressures of 200 psig or even lower. Typical uses for low pressure steam are found in chemical plants and petroleum refineries, where large quantities are used for process heat. Therefore, by joining the combined gas-steam turbine cycle with co-generation of moderate to low pressure steam for process heat, thermal efficiency can be maximized at the hot as well as the cold ends of the cycle with resultant minimum thermal impact on the environment.
Clearly, it would be desirable to locate a chemical plant or a petroleum refinery close or adjacent to a power plant so as to reduce the distance over which steam would need to be transported. But in addition to steam, chemical plants and petroleum refineries can utilize "syngas"--a gas which normally contains carbon monoxide and hydrogen, free from sulfur compounds. Such "syngas" may be utilized directly, or converted to hydrogen by shift conversion of the carbon monoxide and extraction of carbon dioxide.
The combined operation of power generation and chemical plant or petroleum refinery operation provides important opportunities such as:
1. Significantly improved thermal efficiency PA1 2. Improved process economics because of larger scale in the gasification and gas purification plants PA1 3. Improved overall economics from increased efficiency and savings in capital PA1 4. Reduced environmental impact PA1 1. Provide a physical absorption column for extraction of acid gases from a raw syngas rich in CO.sub.2 and containing H.sub.2 S. PA1 2. Remove a part of the rich solution from the absorption column above the H.sub.2 S absorption zone, and return the balance of the rich solution to the column to provide solvent for the removal of H.sub.2 S. PA1 3. Deliver the CO.sub.2 -rich solvent from the absorption column to a second absorption column designed to extract H.sub.2 S from the CO.sub.2 -lean fuel gas. PA1 1. CO.sub.2 is stripped from the rich solution by the CO.sub.2 -lean gas. This CO.sub.2 is delivered into fuel gas at pressure, and enhances the potential for power from a gas turbine because of increased mass flow, or by saving power needed to compress excess combustion air needed to hold combusted gas temperature below a maximum limit determined by the materials used in the turbine. PA1 2. Since part of the CO.sub.2 is delivered into fuel gas, less is absorbed in the solvent, and therefore less is stripped from the solvent with the H.sub.2 S. The concentration of H.sub.2 S in the acid gas delivered to the sulfur (Claus) plant is increased, thus raising the efficiency of the Claus plant. Simultaneously, regeneration energy demand is reduced in the acid gas extraction plant. PA1 3. Since some of the solvent is used twice, total circulation of solvent is reduced, thus saving pumping power. PA1 1. U.S. Pat. No. 3,824,766 issued July 23, 1944 to W. Luley and J. Valentine, assigned to Allied Corp. PA1 2. U.S. Pat. No. 4,332,598 issued June 1, 1982 to R. J. Allam, I. A. Antonas and W. P. Hegarty, assigned to Air Products and Chemicals. PA1 1. A reduced total circulation rate of solvent because at least a part of the solvent is effectively used twice; PA1 2. Increased potential power output from the gas turbine because the CO.sub.2 in the gas contributes to mass flow and requires less compression of excess air for control of combusted gas temperature; PA1 3. Reduced potential release of NO.sub.x to the environment because less air is needed for the gas turbine, and thus leading to lower nitrogen content of the combustion gas; and PA1 4. Improved potential recovery of sulfur from the acid gas feed to a Claus plant because of incrementally higher H.sub.2 S content of the feed gas to the Claus plant, resulting from diversion of part of the CO.sub.2 to the turbine fuel.